The development of technologies to determine the position of a mobile device has enabled application developers and wireless network operators to provide location-based and location-aware services. Examples of these are guiding systems, shopping assistance, friend finder, presence services, community and communication services and other information services that give the mobile user information about his or her surroundings or that use this information to enhance their services.
In addition to the commercial services facilitated by these technologies, location-based emergency services are also being deployed. The governments in several countries have put specific requirements on the network operators to be able to determine the position of an emergency call. For instance, governmental requirements in the United States specify that mobile networks must be able to determine the position of a certain percentage of all emergency calls and further include accuracy requirements. The requirements make no distinctions between indoor and outdoor environment.
In many environments, the position can be accurately estimated by using positioning methods based on Global Navigation Satellite Systems (GNSS), such as the well-known Global Positioning System (GPS). However, GPS-based positioning may often have unsatisfactory performance, especially in urban and/or indoor environments. Complementary positioning methods may also be provided by a wireless network to augment GPS technology. In addition to mobile terminal-based GNSS (including GPS), the following methods are currently available or will be soon be included in the Long-Term Evolution (LTE) standards developed by the 3rd-Generation Partnership Project (3GPP):                Cell ID (CID),        E-CID, including network-based angle-of-arrival (AoA),        Assisted-GNSS (A-GNSS), including Assisted-GPS (A-GPS), based on satellite signals,        Observed Time Difference of Arrival (OTDOA),        Uplink Time Difference of Arrival (UTDOA)—currently being standardized.        
Several positioning techniques are based on time-difference-of-arrival (TDOA) or time-of-arrival (TOA) measurements. Examples include OTDOA, UTDOA, GNSS, and Assisted-GNSS (A-GNSS). A typical, though not the only, format for the positioning result with these techniques is an ellipsoid point with an uncertainty circle/ellipse/ellipsoid, which is the result of intersection of multiple hyperbolas/hyperbolic arcs (e.g., OTDOA or UTDOA) or circles/arcs (e.g., UTDOA, GNSS, or A-GNSS).
Several techniques, such as Adaptive Enhanced Cell Identity (AECID), may involve a mix of any of the methods above, and are thus regarded as “hybrid” positioning methods. With these methods, the position result can be almost any shape, but in many cases it is likely to be a polygon.
Cellular-based positioning methods (as opposed to satellite-based methods, for example) rely on knowledge of anchor nodes' locations, i.e., the fixed locations from which measured signals are transmitted (e.g., for OTDOA) or the fixed locations at which signals transmitted by mobile devices are measured (e.g., for UTDOA). These fixed locations may correspond, for example, to base station or beacon device locations for OTDOA, Location Measurement Unit (LMU) antenna locations for UTDOA, and base station locations for E-CID. The anchor nodes' location may also be used to enhance AECID, hybrid positioning, etc.
Observed Time-Difference-of-Arrival (OTDOA) Positioning
With OTDOA, a terminal measures the timing differences for downlink reference signals received from multiple distinct locations. For each measured neighbor cell, the terminal, referred to as “user equipment” or “UE,” in 3GPP terminology, measures a Reference Signal Time Difference (RSTD). This RSTD is the relative timing difference between transmissions received at the UE from neighbor cell and transmissions received from the reference cell. The UE position estimate can then be found as the intersection of hyperbolas corresponding to the measured RSTDs. Measurements on signals transmitted from three geographically dispersed base stations are needed to solve for two coordinates of the terminal and the receiver clock bias. These base stations should have a good geometry relative to the UE, i.e., each should have a substantially different azimuth from the others.
In order to solve for the UE's position, precise knowledge of the transmitter locations and transmit timing offset is needed. Position calculation can be conducted, for example, by a positioning server, such as the Enhanced Serving Mobile Location Centre (E-SMLC) in LTE), or by the UE. The former approach is known as the UE-assisted positioning mode, while the latter is referred to as the UE-based positioning mode.
To enable positioning in LTE and to facilitate positioning measurements of a proper quality and for a sufficient number of distinct locations, new physical signals dedicated for positioning have been introduced in the LTE standards. These new physical signals are known as positioning reference signals (PRS). In addition, low-interference positioning subframes have been specified in 3GPP.
PRS are transmitted from one antenna port of an LTE eNodeB according to a pre-defined pattern. A frequency shift, which is a function of the physical cell identifier (PCI), can be applied to the specified PRS patterns to generate orthogonal patterns. These techniques result an effective frequency reuse of six, which makes it possible to significantly reduce neighbor cell interference on the measured PRS and to thus improve positioning measurements.
Although PRS have been specifically designed for positioning measurements and in general are characterized by better signal quality than other reference signals, the current standards do not mandate using PRS for downlink positioning measurements. Other reference signals, e.g. cell-specific reference signals (CRS) also could be used for positioning measurements, in principle.
Because PRS signals from multiple distinct locations need to be measured for OTDOA positioning, the UE receiver may have to deal with PRS that are much weaker than those received from the serving cell. Without approximate knowledge of when the measured signals are expected to arrive in time and what is the exact PRS pattern, the UE would need to perform a signal search across a large window, which would impact the time and accuracy of the measurements as well as the UE complexity. To facilitate UE measurements, the network thus transmits assistance data to the UE. This assistance data includes, among other things, a neighbor cell list with PC's, the number of consecutive downlink subframes containing PRS, the PRS transmission bandwidth, etc.
OTDOA Measurement Quality
To facilitate position calculation and an estimation of the quality of the calculated position, some quality metric for the positioning measurements themselves is necessary. This estimated quality is delivered to the network element that uses the positioning measurements, i.e., a positioning node in the network in the case of UE-assisted positioning, or the mobile terminal in the case of UE-based positioning.
Using the standard deviation of several measurements as a measurement quality metric is one common approach in the research literature. This approach has also been standardized for other systems, e.g., for positioning measurements in Universal Terrestrial Radio Access (UTRA) systems. However, in practice this approach has some practical issues, such as an insufficient number of samples to achieve a reliable statistical confidence level. Further, this approach does not allow a system to distinguish poor measurement performance that is due to poor UE receiver implementation from poor measurement performance arising from difficult radio environment conditions.
In International Patent Application Publication WO/2011/105946, published 1 Sep. 2011, a mapping-based approach is described. This approach exploits the relation between a timing measurement, channel characteristics, and a quality metric. The mapping may be in the measuring node (e.g., a UE) or in a network node (e.g., E-SMLC). Another proposal has been to use not an absolute but a relative metric, which is in some respects similar to a method used for testing Channel Quality Indicator (CQI) reporting. With this approach, for a given (i.e., fixed) condition a reported median value is determined, and then it is determined whether a sufficient number of reports fall within a predetermined range. The disadvantage of this approach is that this technique determines only how stable the reported measurements are, but does not indicate the measurements' absolute quality. In particular, the median value for the measurement may be poor, due to poor UE receiver implementation. This problem cannot be detected with this relative metric.
As noted above, the 3GPP specifications for UTRA specify that a UE performing OTDOA measurements must report a positioning quality metric, which is based on the standard deviation of the OTDOA measurements made by the device. In contrast, current specifications for the Enhanced Universal Terrestrial Radio Access Network (E-UTRAN), otherwise known as the LTE network, specify a reporting mapping similar to that used for UTRA, including a “referenceQuality” information element for the reference signal, a “rstd-Quality” information element for each neighbor cell, and an “OTDOA-MeasQuality” information element for each reported measurement, but do not yet specify a specific approach for determining the quality metric. Table 1 below reproduces the field descriptions applicable to the “OTDOA-MeasQuality” information element, as specified in “Evolved Universal Terrestrial Radio Access (E-UTRA); LTE Positioning Protocol (LPP), Release 10,” 3GPP TS 36.355, v. 10.4.0 (December 2012).
TABLE 1UE positioning OTDOA quality in E-UTRAOTDOA-MeasQuality field descriptionserror-ResolutionThis field specifies the resolution R used in error-Value field.The encoding on two bits is as follows:‘00’5 meters‘01’10 meters‘10’20 meters‘11’30 meters.error-ValueThis field specifies the target device's best estimate of the uncertaintyof the OTDOA (or TOA) measurement.The encoding on five bits is as follows:‘00000’0 to(R*1-1) meters‘00001’ R*1 to(R*2-1) meters‘00010’R*2 to(R*3-1) meters. . .‘11111’R*31 meters or more;where R is the resolution defined by error-Resolution field.E.g., R = 20 m corresponds to 0-19 m, 20-39 m, . . . , 620+ m.error-NumSamplesIf the error-Value field provides the sample uncertainty of the OTDOA(or TOA) measurement, this field specifies how many measurementshave been used by the target device to determine this (i.e., sample size).Following 3 bit encoding is used:‘000’Not the baseline metric‘001’5-9‘010’10-14‘011’15-24‘100’25-34‘101’35-44‘110’45-54‘111’55 or more.In case of the value ‘000’, the error-Value field contains the target device's best estimate of the uncertainty of the OTDOA (or TOA) measurement not based on the baseline metric. E.g., other measurements such as signal-to-noise-ratio or signal strength can be utilized to estimate the error-Value.If this field is absent, the value of this field is ‘000’.Positioning Architecture
In 3GPP, location-based services are known as Location Services (LCS). Three key network elements in an LTE positioning architecture are the LCS Client, the LCS target and the LCS Server. The LCS Server is a physical or logical entity that manages positioning for a LCS target device by collecting measurements and other location information, assists the target device in measurements when necessary, and estimating the LCS target location. A LCS Client is a software-based and/or hardware entity that interacts with a LCS Server for the purpose of obtaining location information for one or more LCS targets, i.e., the entities being positioned. LCS Clients may reside in a network node, an external node (i.e., a network external to a cellular network), a Public Safety Access Point (PSAP), a user equipment (or “UE,” 3GPP terminology for an end-user wireless station), a radio base station, etc. In some cases, the LCS Client may reside in the LCS target itself. An LCS Client (e.g., an external LCS Client) sends a request to LCS Server (e.g., a positioning node) to obtain location information. The LCS Server processes and services the received requests and sends the positioning result (sometimes including a velocity estimate) to the LCS Client.
In some cases, the position calculation is conducted by a positioning server, such as an Enhanced Serving Mobile Location Center (E-SMLC) or a Secure User-Plane Location (SUPL) Location Platform (SLP) in LTE. In other cases, the position calculation is carried out by the UE. The latter approach is known as the UE-based positioning mode, while the former approach includes both network-based positioning, i.e., position calculation in a network node based on measurements collected from network nodes such as LMUs or eNodeBs, and UE-assisted positioning, where the position calculation in the positioning network node is based on measurements received from UE.
LTE Positioning Protocol (LPP) is a positioning protocol for control plane signaling between a UE and an E-SMLC, which is used by the E-SMLC to provide assistance data to the UE and by the UE for reporting measurements to the E-SMLC. LPP has been designed in such a way that it can also be utilized outside the control plane domain such as in the user plane in the context of SUPL. LPP is used for DL positioning.
LTE Positioning Protocol Annex (LPPa), sometimes referred to as LTE Positioning Protocol A, is a protocol between the eNodeB and the E-SMLC, and is specified only for control-plane positioning procedures, although it still can assist user-plane positioning by querying eNodeBs for information (e.g., PRS configuration in a cell for OTDOA or UE SRS configuration for UTDOA) and/or eNodeB measurements. LPPa may be used for DL positioning and UL positioning.
FIG. 1 illustrates the UTDOA architecture currently under discussion in 3GPP, including nodes found in the Radio Access Network (RAN) and the core network, as well as an external LCS Client. Although uplink (UL) measurements may in principle be performed by any radio network node, such as the illustrated LTE eNodeB 110, the UL positioning architecture also includes specific UL measurement units, known as Location Measurement Units (LMUs), which are logical and/or physical nodes that measure signals transmitted by a target UE, such as the UE 130 illustrated in FIG. 1. Several LMU deployment options are possible. For example, referring to FIG. 1, LMU 120a is integrated into eNodeB 110, while LMU 120b shares some equipment, e.g., at least antennas, with eNodeB 110. LMU 120c, on the other hand, is a standalone physical node comprising its own radio components and antenna(s).
While the UTDOA architecture is not finalized, there will likely be communication protocols established for communications between a LMU and positioning node, and there may be some enhancements to support UL positioning added to the existing LPPa or to similar protocols.
In particular, a new interface between the E-SMLC and LMU is being standardized for uplink positioning. This interface, known as SLm, is terminated between a positioning server, e.g., the E-SMLC 140 pictured in FIG. 1, and an LMU. It is used to transport messages according to the SLmAP protocol, a new protocol being specified for UL positioning, between the E-SMLC and the LMU. SLmAP can be used to provide assistance data to an LMU, as discussed in further detail below. This protocol may also be used by the LMU to report to the E-SMLC results of measurements on radio signals performed by the LMU. The SLmAP protocol was previously referred to as the LMUp protocol; thus it should be understood that references herein to SLmAP are referring to a developing protocol referred to as LMUp elsewhere.
In LTE, UTDOA measurements, known as UL relative time-of-arrival (RTOA) measurements, are performed on Sounding Reference Signals (SRS). To detect an SRS signal, an LMU 120 needs a number of SRS parameters to generate an SRS sequence that is correlated against the received signal. These parameters are not necessarily known to LMU 120. Thus, to allow the LMU to generate the SRS sequence and detect the SRS signals transmitted by a UE, SRS parameters must be provided in the assistance data transmitted by the positioning node to LMU; these assistance data would be provided via SLmAP. However, the SRS parameters are also generally unknown to the positioning node, which therefore must obtain this information from the eNodeB that configured the target UE to perform the SRS transmissions to be measured by the LMU; this information would have to be provided to the positioning node via LPPa or a similar protocol. Table 2 illustrates examples of parameters that might be signaled from an eNodeB to E-SMLC, using LPPa, for example.
The specific contents of the assistance data provided to LMUs by a positioning node, over SLmAP, are currently being discussed in 3GPP. One intention of the assistance data is to indicate the SRS configuration for the uplink signals that the LMUs will measure. One example of the specific assistance data that might be provided to an LMU by a positioning node, using SLmAP, is shown in Table 3. This assistance data, which can be based on information provided to the E-SMLC by an eNodeB, can be used by the LMU to configure UL RTOA measurements, for example.
TABLE 2Parameters that may be signaled from eNodeB to E-SMLC,e.g., over LPPaParameterCategoryParametersGeneralPCI of PCellNote 1UL-EARFCN of PCellTiming advance measurement for the UE inPCell [36.214]Note 3SRSFor each serving cell in which SRS is configuredNote 2:PCIUL-EARFCNduplex modeUL cyclic prefixUL system bandwidth of the cellCell-specific SRS bandwidth configurationsrs-BandwidthConfig [36.211]UE-specific SRS bandwidth configurationsrs-Bandwidth [36.211]number of antenna ports for SRS transmissionsrs-AntennaPort [36.211]frequency domain position [36.211]SRS frequency hopping bandwidth configuration [36.211]SRS-Cyclic shift [36.211]Transmission comb [36.211]SRS configuration index [36.213]MaxUpPt, used for TDD only [36.211]Group-hopping-enabled [36.211]deltaSS, parameter Δss [36.211, 5.5.1.3], includedwhen SRS sequence hopping is used [36.211, 5.5.1.4] andnot included otherwiseNote 1Indicating PCell should not imply configuring SRS on the PCellNote 2Multiple serving cells are possible for a UE configured with CANote 3Used for search window calculation
TABLE 3Parameters that may be signaled from E-SMLC to LMU,e.g., over SLmAPParameterCategoryParametersGeneralSearch window parametersNote 2:expected propagation delay, T, corresponding todistance between LMU and PCell,delay uncertainty ΔSRSFor each serving cell in which SRS is configuredand to be measured by LMUNote 1:PCIUL-EARFCNduplex modeUL cyclic prefixUL system bandwidth of the cellCell-specific SRS bandwidth configurationsrs-BandwidthConfig [36.211]UE-specific SRS bandwidth configurationsrs-Bandwidth [36.211]number of antenna ports for SRS transmissionsrs-AntennaPort [36.211]frequency domain position [36.211]SRS frequency hopping bandwidth configuration [36.211]SRS-Cyclic shift [36.211]Transmission comb [36.211]SRS configuration index [36.213]MaxUpPt, used for TDD only [36.211]Group-hopping-enabled [36.211]deltaSS, parameter Δss [36.211, 5.5.1.3], includedwhen SRS sequence hopping is used [36.211, 5.5.1.4] andnot included otherwiseNote 1Multiple serving cells are possible for a UE configured with CA; SRS may be not transmitted on allNote 2Search window is calculated by the LMU as [T − Δ, T + Δ], where Δ may be e.g. the timing advance measurement provided by eNodeB
Since the eNodeB is configuring UE transmissions in general, including the SRS transmissions, it has to communicate to the positioning node the configuration information for the UL transmissions to be measured for UL positioning. It has been proposed that the same configuration information signaled to LMUs by the positioning node is proposed to be also signaled from the eNodeB to the E-SMLC.
Measurements for UL positioning and UTDOA are performed on UL transmissions, which may include, for example, reference signal transmissions or data channel transmissions. UL RTOA is the currently standardized UTDOA timing measurement, and may be performed on Sounding Reference Signals (SRS). The results of the measurements are signaled by the measuring node (e.g., LMU) to the positioning node (e.g., E-SMLC), e.g., over SLmAP.
FIG. 2 illustrates the current architecture under discussion in 3GPP for downlink (DL) positioning, again including nodes found in the Radio Access Network (RAN) and the core network, as well as an external LCS Client. It will be appreciated that this architecture includes many of the same components found in the UL positioning architecture illustrated in FIG. 1. Two additional components shown in FIG. 2, however, are the Serving Gateway (S-GW) and the Packet Data Network Gateway (PDN GW, or P-GW). These gateways terminate the UE's interfaces towards the E-UTRAN network and the Packet Data Network (PDN), respectively.
As noted above, in the current standards for OTDOA positioning, RSTD measurement quality reporting for a neighbor cell is mandated, but no method is specified. This leads to several problems. First, not all wireless devices may report a reliable quality metric for RSTD measurements. Second, different algorithms for estimating the RSTD quality in different UEs may lead to reporting different values even in the same radio environment, same radio signal quality, and even with the similar receiver capabilities. Unreliable RSTD measurement quality will negatively impact the accuracy of the calculated position. More generally, terminal-based quality reporting is subject to channel variations in time. As a result, reported quality may not be precise, depending on the prevailing channel conditions. Again, this may have a significant impact of the accuracy of calculated position. Accordingly, improved techniques for assessing and reporting measurement qualities for downlink positioning measurements are needed.